Drones and crawling robots outfitted with special scanning technology could help wind blades stay in service longer, which may help lower the cost of wind energy at a time when blades are getting bigger, more expensive and harder to transport, Sandia National Laboratory researchers say. As part of the Department of Energy’s Blade Reliability Collaborative work, funded by the Wind Energy Technologies Office, Sandia researchers partnered with energy businesses to develop machines that non-invasively inspect wind blades for hidden damage while being faster and more detailed than traditional inspections with cameras.

“Wind blades are the largest single-piece composite structures built in the world, even bigger than any aircraft, and they often get put on machines in remote locations,” said Joshua Paquette, a mechanical engineer in Sandia’s wind energy program. “A blade is subject to lightning, hail, rain, humidity and other forces while running through a billion load cycles during its lifetime, but you can’t just land it in a hanger for maintenance.”

Sandia is drawing on expertise from avionics and robotics research to change that by catching damage before it becomes visible. “Smaller and cheaper repairs can fix the blade and extend its service life” says Joshua. In one project, Sandia outfitted a crawling robot with a scanner that searches for damage inside wind blades. In a second series of projects, Sandia paired drones with sensors that use the heat from sunlight to detect damage.

Inspecting, Repairing Wind Blades In The Field Presents Big Challenge

“Traditionally, the wind industry has had two main approaches to inspecting wind blades”, states Joshua. The first option is to send someone out with a camera and telephoto lens. The inspector moves from blade to blade snapping photos and looking for visible damage, like cracks and erosion. The second option is similar but instead of standing on the ground, the inspector rappels down a wind blade tower or maneuvers a platform on a crane up and down the blade.

“In these visual inspections, you only see surface damage,” Joshua said. “Often though, by the time you can see a crack on the outside of a blade, the damage is already quite severe. You’re looking at a very expensive repair, or you might even have to replace the blade.”

These inspections have been popular because they are affordable, but they miss out on the opportunity to catch damage before it grows into a larger problem, he said. Sandia’s crawling robots and drones are aimed at making noninvasive internal inspection of wind blades a viable option for the industry.

Crawling Robot Finds Hidden Damage

Sandia and partners International Climbing Machines and Dophitech built a crawling robot inspired by the machines that inspect dams. The robot can move from side to side, up and down a wind blade, like someone mowing a lawn. On-board cameras provide real-time, high-fidelity images to detect surface damage, as well as small demarcations that may signal larger, subsurface damage. While moving, the robot also uses a wand to scan the blade for damage using phased array ultrasonic imaging.

The scanner works much like the ultrasound machines used by doctors to see inside bodies, except in this case it detects internal damage to blades by sending back a series of signals. Changes in these ultrasonic signatures can be automatically analyzed to indicate damage. Sandia senior scientist and robotic crawler project lead Dennis Roach said that a phased array ultrasonic inspection can detect damage at any layer inside the thick, composite blades.

“Impact or overstress from turbulence can create subsurface damage that is not visually evident,” he said. “The idea is to try to find damage before it grows to critical size and allow for less expensive repairs that decrease blade downtime. We also want to avoid any failures or the need to remove a blade.”

Dennis envisions the robotic crawlers as part of a one-stop inspection and repair solution for wind blades.

“Picture a repair team on a platform going up a wind blade with the robot crawling ahead,” he said. “When the robot finds something, remotely located inspectors can have the robot mark the spot so that the location of subsurface damage is evident. The repair team will grind away the damage and repair the composite material. This one-stop shopping of inspection and repair allows the blade to be put back into service quickly.”

Drones Use Heat from sunlight to reveal blade damage

Sandia worked with several small businesses in a series of projects to outfit drones with infrared cameras that use the heat from sunlight to detect hidden wind blade damage. This method, called thermography, can detect damage up to a half inch deep inside the blade.

“We developed a method to heat the blade in the sun, and then pitch it into the shade,” Sandia mechanical engineer Ray Ely said. “The sunlight diffuses down into the blade and equalizes. As that heat diffuses, you expect the surface of the blade to cool. But flaws tend to disrupt the heat flow, leaving the surface above hot. The infrared camera will then read those hot spots to detect damage.” Ground-based thermography systems are currently used for other industries, such as aircraft maintenance. Because the cameras are mounted on drones for this application, concessions have to be made, Ray said.

“You don’t want something expensive on a drone that could crash, and you don’t want a power hog,” he said. “So, we use really small infrared cameras that fit our criteria and use optical images and lidar to provide additional information.”

Lidar, which is like radar but with light instead of radio frequency waves, measures how long it takes light to travel back to a point to determine the distance between objects. Taking inspiration from NASA’s Mars lander program, the researchers used a lidar sensor and took advantage of drone movement to gather super-resolution images. A drone inspecting a wind blade moves while it takes images, and that movement makes it possible to gather a super-resolution image.

“You use the movement to fill in additional pixels,” he said. “If you have a 100 by 100-pixel camera or lidar and take one picture, that resolution is all you’ll have. But if you move around while taking pictures, by a sub-pixel amount, you can fill in those gaps and create a finer mesh. The data from several frames can be pieced together for a super-resolution image.”

Using lidar and super-resolution imaging also makes it possible to precisely track where the damage on a blade is, and lidar can also be used to measure erosion on blade edges.

Autonomous Inspections Are The Future

Autonomous inspections of bridges and power lines are already realities, and Joshua believes they also will become important parts of ensuring wind blade reliability. “Autonomous inspection is going to be a huge area, and it really makes sense in the wind industry, given the size and location of the blades,” Joshua said. “Instead of a person needing to walk or drive from blade to blade to look for damage, imagine if the inspection process was automated.”

Joshua said there is room for a variety of solutions and inspection methods, from a simple ground-based camera inspection, to drones and crawlers, all working together to determine the health of a blade.

“I can envision each wind plant having a drone or a fleet of drones that take off every day, fly around the wind turbines, do all of their inspections, and then come back and upload their data,” Joshua said. “Then the wind plant operator will come in and look through the data, which will already have been read by artificial intelligence that looks for differences in the blades from previous inspections and notes potential issues. The operator will then deploy a robotic crawler on the blade with suspected damage to get a more detailed look and plan repairs. It would be a significant advance for the industry.”

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